Use of Natural Pigment from Orchid as a Reagent for Quantitative Analysis of Ammonia in Chemical Fertilizers

Abstract

In this work, use of natural pigment, crude aqueous extracted from orchid ‘Dendrobium sonia earsakul’, as a reagent for determination of ammonia was investigated.  The extract contained anthocyanin compounds which is red in acidic solution, purple in neutral solution and greenish-yellow in alkaline solution.  The reference method is based on flow injection analysis with gas diffusion unit (GD).  The sample solution was injected and mixed with NaOH in order to generate ammonia gas.  The gas consequently diffused through the PTFE membrane, at GD unit, and finally dissolved into bromothymol blue acceptor resulting in a change of pH.                    Our method proposes using of the orchid extract as acceptor reagent instead of using bromothymol blue.                The change in color of the orchid extract is monitored using a spectrophotometer at 600 nm and correlates with the concentration of ammonia in the sample.  The method shows good linear response with a detection limit of 2.17 mM and quantitation limit of 2.99 mM. Throughput of 48 samples/h was achieved.  The intra-day precision is 0.70% (n=10, 20 mM NH4Cl) and inter-day precision is 2.62% (n=9, 20 mM NH4Cl).  Comparison of sample analysis results obtained from our method and those obtained from reference method will be discussed.   Keywords : ammonia, fertilizer, orchid, flow injection analysis, gas diffusion unit